linux-kernel - Re: [RFC][PATCH] kprobes: Add separate preempt

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Date:	Fri, 01 Jul 2011 10:52:26 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	yrl.pp-manager.tt@...achi.com,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Hans-Christian Egtvedt <hans-christian.egtvedt@...el.com>,
	Russell King <linux@....linux.org.uk>,
	Tony Luck <tony.luck@...el.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	MartinSchwidefsky <schwidefsky@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [RFC][PATCH] kprobes: Add separate preempt_disabling for kprobes

(2011/07/01 10:38), Steven Rostedt wrote:
> [ Added some of the affected maintainers, left off David Howells and
> David Miller due to LKML Cc limit ]
> 
> On Fri, 2011-07-01 at 10:22 +0900, Masami Hiramatsu wrote:
>> (2011/07/01 6:56), Peter Zijlstra wrote:
>>> On Thu, 2011-06-30 at 11:51 -0400, Steven Rostedt wrote:
>>>>
>>>> To solve this, I've added a per_cpu variable called
>>>> kprobe_preempt_disabled, that is set by the kprobe code. If it is set,
>>>> the preempt_schedule() will not preempt the code.
>>
>> Sorry for replying so late :(
> 
> Heh, who can blame you? Timezones make open source development a
> wait-and-see affair.

Yeah, we can't go over light speed.

>>> Damn this is ugly. Can we step back and see if we can make the
>>> requirement for kprobe to disable preemption go away?
>>
>> As I replied right now, I think we can just eliminate that
>> disabling preemption code. At least we'd better try it.
>> I agree with you, introducing this kind of complexity
>> just for kprobes is not what I want. :(
> 
> Note, I did clean up this patch, so it is not as fugly.

Hm, I think you don't need to introduce new flag for that
purpose, there is current_kprobe and kprobe status flag.

if (kprobe_running() &&
    get_kprobe_ctlblk()->kprobe_status == KPROBE_HIT_SS)
	/*Running under kprobe's single stepping*/

But I'm not sure that is there any code which can run
under TF=1. (maybe NMI code? but it would not cause preemption)

>>> Why does it have to do that anyway? Isn't it keeping enough per-task
>>> state to allow preemption over the single step?
>>
>> preemption itself must not happen on single stepping, but it seems
>> impossible to do heavy context switching with setting TF bit...
> 
> Yeah, if all archs single step with interrupts disabled, then we should
> be fine with removing preemption.

OK, I'll check that.

Thank you ;)

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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